AC to DC converters and controlled rectifier circuits, due to the chopping action of their power switches, present a nonlinear load to an AC linear power source causing harmonic currents to be generated which are fed back to the AC linear power source line. These harmonic currents, in conjunction with the AC source impedance, generate voltage components with highly distorted waveforms which appear across and adversely affect the performance of other electronic circuits connected to the power source transmission line. These excessive harmonic currents also cause a low power factor condition, thereby greatly reducing the efficiency of the transmission line. When this power factor condition is caused by a converter with a capacitive input filter, high inrush currents can occur at initial start up of the converter. The adverse effects of these harmonic currents have priorily been compensated for by using filter circuitry to divert the harmonic currents to a ground reference or by generating compensating currents designed to cancel or neutralize the harmonic currents.
More fundamental approaches have involved redesign of the converter or rectifier circuit to modify the nature of its apparent input impedance. An example of such an approach is disclosed in U.S. Pat. No. 3,913,002 issued to R. L. Steigerwald on Oct. 14, 1975, wherein a load current is controlled by comparing it with a reference waveform in phase with the line voltage using a comparator circuit with a defined hysteresis to define switching band limits at the power switches about the reference waveform. This control system shapes the input line current in response to a particular reference waveform selected to obtain a desired power factor which minimizes line current harmonics.
These prior arrangements may be unsuitable in specific applications as a consequence of not having a fixed switching frequency control scheme. For example, since the switching frequency is dependent upon the load characteristics, varying load conditions may cause the frequency to be in the audible range, creating acoustic noise problems. The variable switching frequency may also make high frequency transformer isolation difficult or impractical, especially if the load characteristics are not known.
Although the prior arrangements reduce the line frequency harmonic currents, significant switching frequency harmonic currents may be generated if the topology employed produces discontinuous input currents. This often requires excessive input filtering, especially at high power levels where RMS currents are very large.